Recently, a metallic belt-type continuously variable transmission and a toroidal continuously variable transmission have been developed as a transmission for an automobile and the like and have already been in practical use. Initially, a lubricating oil for an automatic transmission was also used for a continuously variable transmission. However, in accordance with an improvement in performance of the continuously variable transmission, the lubricating oil has been required to have more excellent properties. Particularly, since a friction property of a lubricating oil used for a wet clutch in an automatic transmission has been optimized for an automatic transmission, a friction coefficient between metals of the lubricating oil is likely to be insufficient when the lubricating oil is used for a continuously variable transmission, so that it is difficult to transmit a large volume of torque.
For this reason, various lubricating oils usable for a continuously variable transmission have been developed. For instance, there have been proposed a lubricating oil composition containing (a) alkaline earth metal sulfonate or phenate, (b) an imide compound and (c) a phosphorus compound (see Patent Literature 1) and a lubricating oil composition containing: (A) at least one phosphorus-containing compound selected from phosphoric monoester, phosphoric diester and phosphorus monoester, which each have a hydrocarbon group having 1 to 8 carbon atoms; and (B) a tertiary amine compound substituted by a hydrocarbon group having 6 to 10 carbon atoms (see Patent Literature 2). Moreover, a lubricating oil composition containing (A) a tertiary amine, (B) acid phosphate and the like and (C) metal sulfonate and the like has also been proposed (see Patent Literature 3). The lubricating oil compositions disclosed in these Patent Literatures have a high friction coefficient between metals suitable for a lubricating oil for a continuously variable transmission.